An anastrophe of cum vōbīs (“with ye, with ye”), from vōbīs (“you, ye; plural”) the ablative of vōs (“you, ye; plural”), the plural of tū (“you; singular”) and cum (“with”) where cum is always followed by a personal pronoun. Compare with mēcum (“with me”), derived from cum and mē (“me”), tēcum (“with you”), from cum and tē (“you”), sēcum (“with oneself”), from cum and sē (“oneself”), nōbīscum (“with us”), from cum and nōbīs (“us”).
The odd word ordering on which this contraction is based comes from the fact that in Proto-Indo-European the word *kom (from which cum derives) was special particle, somewhat like an adverb, not a preposition as it became in Latin. As such the *kom could appear before or after the object pronoun since it was the object of the verb, not the object of a preposition. As these special particles evolved into prepositions this word order became archaic even though it was still commonly used. Thus the contraction vobiscum evolved into an adverb in its own right.
- with you all, with ye (where "you" refers to several subjects)